U.S. Pat. No. 4,036,486, incorporated herein by reference, describes a mat board cutter by which a mat board is cut to have a desired peripheral size to fit within a desired frame and to have a desired central opening through which a picture being framed is visible.
As described in greater detail in U.S. Pat. No. 4,036,486, cutting the mat board requires sliding a cutter-carrying body along a guide rail with a knife blade included in a cutter device or assembly pivotably carried on the body in engagement with the mat, which mat is held in a predetermined position by abutment means on a base and a clamp bar on which the guide rail is mounted.
The knife blade included in the cutter assembly is thin and has a truncated pyramidal periphery with a cutting edge along its base, opposite end edges converging from its cutting edge, and a top edge parallel to its cutting edge. Also included in the cutter assembly is a pivot block having a socket receiving the knife blade with an end portion of the knife blade and its cutting edge projecting out of the socket, and means in the form of a thumb screw adapted to bear against one side surface of the knife blade for releasably fixing the knife blade in the socket. Means are provided for mounting the pivot block on the cutter-carrying body for povital movement between a disengaged position with the projecting portion of the knife blade spaced from a mat board on the cutter, and an engaged position with the knife blade engaged with the mat board. With this design, the possibility exists that if the thumb screw is not securely tightened or works loose during use of the mat board cutter, the knife blade can slide from within the socket and move to a position at which it can injure a user.